Connection structure of superconducting layer, superconducting wire, superconducting coil, and superconducting device

A superconducting wire comprising:

• a first superconducting wire including a first superconducting layer;
• a second superconducting wire including a second superconducting layer;
• a third superconducting layer;
• a first connection layer provided between the first superconducting layer and the third superconducting layer and including a crystal region containing a rare earth element (RE), barium (Ba), copper (Cu), and oxygen (O), and a void;
• and
• a second connection layer provided between the second superconducting layer and the third superconducting layer and including a crystal region containing a rare earth element (RE), barium (Ba), copper (Cu), and oxygen (O), and a void, wherein
• in a cross section perpendicular to a surface of the first superconducting layer, the crystal region of the first connection layer includes a path extending from the first superconducting layer to the third superconducting layer,
• the path includes a plurality of narrow portions, and a width of a minimum narrow portion having a minimum width among the plurality of narrow portions is equal to or more than 300 nm, and
• in the cross section, an area ratio of the void of the first connection layer is equal to or more than 30% and equal to or less than 70%.